Due to variation in magnetic forces within rotating permanent magnet machines pulsations of torque occur. The torque pulsations occur at no-load and load situations, hereafter denoted cogging torque and ripple torque, respectively. These torque pulsations may lead to vibrations which can be harmful to the rotating machine and interconnected parts. Furthermore, the torque pulsations can create acoustic noise of low frequency which can disturb the environment. In addition to the torque pulsations caused by tangential forces, radial forces exist creating practically the same problems and at the same frequencies. These issues can be even more pronounced in direct drive wind generators.
Several techniques exist to reduce cogging and ripple torque as well as radial force. The most common are pole shifting, magnet shaping, tooth shaping (including dummy slots) and skew. These techniques have the drawback that the average torque is reduced for a given level of current. Thus, current needs to be increased in order to provide the same torque. Similarly, active ripple reduction can be carried out by superimposing harmonics in the current. This will lead to increased copper losses as well.